Rubber vulcanization process



2,817,694 Patented Dec. 24, 1957 2,817,694 RUBBER VULCANIZATION PROCESSKenneth E; Creed, Jr., Nitro, and John J. DAmico,

Charleston, W. Va., assignors to Monsanto Chemical Company, St. Louis,Mo., a corporation of Delaware No Drawing. Application September 17,1953 Serial No. 380,886

8 Claims. (Cl. 260-787) C -SH where R is an alkyl radical such asmethyl, ethyl, propyl, butyl, amyl, etc., and its salts when employed inconjunction with a vulcanization accelerator, particularly asulfur-containing primary organic vulcanization accelerator, provide fora substantial improvement in processing safety in the compounding ofsulfur-vulcanizable rubbers. Additionally the new vulcanizationaccelerator adjuvants provide for a more efiicient and more economicaluse of sulfur-containing primary organic vulcanization accelerators inthe compounding of sulfur-vulcanizable rubbers reinforced with furnacecarbon blacks such as the high abrasion (HAF) and super abrasion (SAP)furnace blacks.

The new vulcanization accelerator adjuvants of this invention may beprepared by a 'variety of well kh'own methods, as for example byhydrolyzing a 4-alkyl-5;-carbo methoxy-2-thiazolethiol, the latter beingprepared by reacting an ester of the formula R-o-oHo1-o-oon t t where Ris an alkyl radical with ammonium dithiocarbamate in substantiallyequimolecular proportions in an aqueous medium.

A specific illustration of the preparation of the new vulcanizationaccelerator adjuvants is as follows:

Substantially one molecular proportion of methyl chloroacetoacetate andsubstantially one molecular proportion of ammonium dithiocarbamate areintimately mixed in the presence of substantially 55 molecularproportions of water. The reaction is immediate and exothermic and aprecipitate forms. The reaction mass is agitated for about 8 hours andis then allowed to stand to permit settling of the precipitate. Theprecipitate is filtered off, washed with Water until the washings areneutral to litmus. The crude solid, substantially 0.73 molecularproportion of 4-methyl-S-carbomethoxy-Z-thiazo'lethiol, is then admixedwith substantially 1.46 molecular proportions of sodium hydroxide in theform of a 25% by Weight aqueous solution. Substantially 45 molecularproportionsof Water is then added and with constant agitation the mix isagitated for about 2 hours at about 60-70 C. The mass is cooled to about15 C. and acidified with concentrated hydrochloric acid. The precipitateis filtered olf, Washed with water and dried. The light yellow solid soobtained is 2-mercapto-4-methylthiazole-S-earboxylic acid.

The salts contemplated by this invention are readily prepared from thefree mercaptan and as illustrative thereof is the following:

EXAMPLE 2 Substantially 25 parts by Weight of 2-mercapto-4-methyl-thiazole-S-carboxylic acid is admixed with substantially 11.44parts by weight of a 10% aqueous solution of sodium hydroxide andsubstantially 500 parts by Weight of water. To the agitating mass isadded a'solution containing substantially 36.7 parts by weight ofcadmium sulfate dissolved in substantially 900 parts by weight of Water.The mass is then agitated for about 1 hour. The resultant precipitate isthen filtered olf, washed with water and dried. The product is'thecadmium salt of Z-mercapto-4-methyl-thiazole-S-carboxylic acid,

Similarly but replacing cadmium sulfate with an equimolccular amount ofzinc chloride, the zinc salt of 2-mercapto-4-methyl-thiazole-5-carboxylic acid is obtained.

As illustrative of the properties of the new vulcanization acceleratoradjuvants of this invention the following stocks were compoundedcomprising Stock A. B 0

Parts by weight Smoked sheets rubber. 100 I00 Furnace carbon black. 5050 50 Zinc oxide 5. 0 5.0 5.0 Stearic acid .r 2.0 2.0 230 Saturatedhydrocarbon softener 3. 0 3. 0 3.0 Sulfur 2t 5 2. 5 2. 5Z-Mercaptobenzothiazole c 0. 8 O. 2 2- M'ercapto-- methyl-thiazolcarboxylic acid 0. 8 0. 6

The above stocks so compounded were cured in the usual manner by heatingin a press for different periods of time at 144 C. The physicalproperties of the respective cured stocks are set forth below:

Table 1 Modulus of Time of Elasticity Tensile at Ultimate Stock Cure inin lbs/in. at Break'in Elongation,

Mins. Elongation lbsJin. percent 30 2,063 3, 486 A63 30 1 1, 056 2,096 473 30 1,683 3,060 470 6D 1, 965 3, 230 435 6D 1, 295 2, 310 j {159 60 1,745 2,915 I 450 The scorch times as determined by the Mooney 3 5,.tometer at 1 21 C. were found to be as follo As further illustrative ofthe properties of the newvulcanization accelerator adjuvants of thisinvention, a base rubber stock was compounded comprising:

Parts by weight Employing the above base stock the following compositesof new vulcanization accelerator adjuvant and vulcanization acceleratorwere separately incorporated and compared to a stock vulcanized in thepresence of the new vulcanization accelerator adjuvant alone:

Parts by Additive Weight 2-Mercapto-4-methyi-thiazole-5-carboxylic acid.

The above stocks so compounded were cured in the usual manner by heatingin a press for different periods of time at 144 C. The physicalproperties of the respective cured stocks are set forth below:

Table III Modulus of Time of Elasticity Tensile at; Ultimate Stock Curein in lbs./in. at Break in Elongation,

Minutes Elongation lbs/1.11. percent 4 a I Table IV Scorch time inminutes As will be obvious to those skilled in the art upon perusing theforegoing data this invention provides for substantial improvements inthe compounding of sulfurvulcanizable rubbers. Other vulcanizationaccelerators may be employed than the afore-described such as2-mercapto-N-ethyland 2-mercapto-N-isopropyl-benzothiazolesulfenamide,Z-mercapto-N-morpholinyland 2-rnercapto-N-piperidyl-benzothiazolesulfenamide,Z-rnercapto-N-ethyl-4,5-dimethyl-thiazolesultenamide, and likethiazolesulfenamide accelerators. Additionally such thiuram sulfides astetramethyl thiuram disulfide as well as the tetraethyl-, tetrapropylandtetrabutyl thiuram monoand di-sulfides, etc. and also thel,2-bis(thiazolylmercaptomethyl)urea vulcanization accelerators,particularly the reaction products of mercaptoarylenethiazoles withmethylol carbamides and methylol'thiocarbamides as described anddisclosed by Robert L. Sibley in U. S. Patent 2,145,808 may be employedin conjunction with the new vulcanization accelerator adjuvants of thisinvention.

As. further illustrative of this invention the following stocks werecompounded comprising P Q R S '1 Stock Parts by weight Smoked sheetsrubber 100 100 100 100 Furnace carbon black 50 50 50 50 50 Stearicucid--. 2. 0 2.0 2. 0 2.0 2.0 Zinc oxide 5.0 5.0 5.0 5.0 5.0 Saturatedhydrocarbon softener 3. 0 3.0 3.0 3.0 3.0 Sulfur 2. 5 2. 5 2. 5 2. 5 2.5 2-Mercapto-4-1nethyl-thiazole 5-carboxylic acid 0. 8 0. 6 0. 6 0 6 0.8Diphenyl guanidine 0.2 Butyraldehyde-aniline condensation produ 0.2Tetramethyl thiuram disulfide 0. 2 Lead oxide 1. 5

The above stocks so compounded were cured in the usual manner by heatingin a press for different periods of time at 144 C. The physicalproperties of the respective cured stocks are set forth below:

The scorch times as determined by the Mooney plastometer at 121 C. werefound to be as follows:

Table V Modulus of Time of Elasticity in Tensile at Ultimate Stock Curein lbs/111. at Break in Elongation,

Minutes Elongation oi lbs/in. Percent The scorch times as determined bythe Mooney Plastometer' at 12 1 C. were'tfound to be as follows Table VIStock: Scorch time in minutes l 30 Q 1 R 17 S l1 As further illustrativeof the properties of the new vulcanization accelerator adjuvants of thisinvention the 01 lowing butyl rubber stocks were compounded comprisingStOOk'.

Butylrubberi Carbon black-n The above stocks so compounded were cured inthe usual manner by heating in a press for different periods of time at330 F. The physical properties of the respective cured stocks are setforth below:

The scorch time as determined by the Mooney Plastorneter employing thelarge rotor at 135 C. was found to be in excess of 30 minutes for therespective stocks.

As further illustrative of the properties of the new vulcanizationaccelerator adjuvants of this invention, the following gum stocks werecompounded comprising Stock W X Parts by weight Zinc oxid Stearic acidSulfur Diphenyl guan 2-Mercapt0-4methyl-thiazole-5-carboxyllc a Smokedsheets rubber The above stocks so compounded were cured in the usualmanner by heating in a press for different periods of time at 135 C. Thephysical properties of the respective cured stocks are set forth below:

Table VIII Modulus of Elasticity in Tensile Ultimate Time of lbs/111. atElongations 0l at Elonga- Stock Cure in Break tion,

Minutes in Percent 300% 500% 700% lbs/in! 45 .i no cure 45 110 230 8162, 256 900 60 no cure 60 130 320 1, 175 2, 780 885 90 no cure 90 196 4701, 656 3, 140 810 tometer employing the large rotor at 121 C. for stoekx was found to be 24 minutes. y

In place of diphenyl guanidine inthe aforedescribed gum stocks otherbasic di-arylene guanidines such as di otolyl guanidine, di-oethylphenylguanidine andthe like in admixture with the new vulcanizationaccelerator adjuvants of this invention provide similar processingsafety in the compoundingof sulfur-vulcanizabl'e rubbers,

By the term sulfunvulcanizable rubbers 'as employed in the specificationand appended claims is meant toilicl'ude natural as well as "syntheticrubbers which are capable of vulcanization when heated with sulfur andincludes latices and reclaims of such materials whether or not admixedwith pigments, fillers, softeners; antioxidants, etc.

By the term vulcanization accelerator as employed in the specificationand appended claims is meant the various well-known chemicals employedfor accelerating'or catalyzing the sulfur-cure of a vulcanizable rubberwhether natural or synthetic and it is to be understood that said termincludes the inorganic accelerators such "as lead oxide and the organicaccelerators suchas thearomatic and non-aromatic thiazole sulfides, thethiazolesulfenamides, and the other thiazole vulcanization accelerators,the various dithi'ocarbamates, the thiuram monoand disulfides, thealdehydeamine accelerators, the various basic di-arylene guanidineaccelerators and the like. A vulcanization accelerator mixture of atetraalkyl thiuram sulfide, such as tetramethyl thiuram monosulffide,and a l,2-bis(thiazolyl mercaptomethyl)urea, such as 1,2-bis(2-benzothiazolyl mercaptomethyl)urea, supplemented by the newvulcanization accelerator adjuvant of this invention provides for ahighly useful composite in the compound ing of sulfur-vulcanizablerubbers for mechanical goods, especially from the standpoint ofprocessing safety, and represents a particularly preferred embodiment ofthis invention.

While certain embodiments of this invention have been described indetail it is to be understood that this invention is not so limited butthat variations and modifications obvious to those skilled in the art,as for example substituting materials having equivalent chemicalproperties and varying the proportion of the new vulcanizationaccelerator adjuvants with respect to the vulcanization accelerator maybe made depending upon the type of vulcanization accelerator employed,the type of sulfurvulcanizable rubber, the designs of the rubbercompounder, etc., without departing from the spirit or scope of theinvention.

What is claimed is:

1. The method of vulcanizing a sulfur-vulcanizable rubber whichcomprises heating a sulfur-vulcanizable hydrocarbon rubber, sulfur, anda mixture of different accelerators one of which is 014.0 parts byweight based on the rubber hydrocarbon of a compound selected from thegroup consisting of a mercaptan and its salts, said mercaptan having thestructure where R is an alkyl radical the other accelerating componentbeing employed in a proportion within the range of 0.1-2.0 parts byweight of the rubber hydrocarbon.

2. The method of vulcanizing a sulfur-vulcanizable rubber whichcomprises heating a sulfur-vulcanizable hydrocarbon rubber, sulfur, anda mixture of different organic accelerators one of which is 0.14.0 partsby weight based on the rubber hydrocarbon of aZ-mercaptol-alkyl-thiazole-S-carboxylic acid the other acceleratingcomponent being employed in a proportion within the '7 range of 0.1-2.0parts by weight of the rubber hydrocarbon.

3. The method of vulcanizing a sultur-vulcanizable rubber whichcomprises heating a sulfur-vulcanizable hydrocarbon rubber, sulfur,furnace carbon black, and a mixture of different organic acceleratorsone of which is 0.1-4.0 parts by weight based on the rubber hydrocarbonof a 2-mercapto4-alkyl-thiazole-5-carboxylic acid the other acceleratingcomponent being employed in a proportion within the range of 0.1-2.0parts by Weight of the rubber hydrocarbon.

- ,4. The method of vulcanizing natural rubber whichcomprises heatingnatural rubber, sulfur, and a mixture of different organic acceleratorsone of which is 0.1-4.0 parts, by weight based on the rubber hydrocarbonof 2- mercapto-4-methyl-thiazole-5-carboxy1ic acid the other arecelerating component being employed in a proportion within the range of0.1-2.0 parts by weight of the rubber hydrocarbon.

- 5'. The method of vulcanizing natural rubber which comprises heatingnatural rubber, sulfur, 0.1-0.6 part by weight of the rubber hydrocarbonof a di-aryl guanidine vulcanization accelerator and 0.1-0.7 part byweight of the rubber hydrocarbon of 2-mercapto-4-rnethylthiazole-S-carboxylic acid.

, 6. The method of vulcanizing natural rubber which comprises heatingnatural rubber, sulfur, a vulcanization accelerator mixture of 005-01part by weight of the rubmercaptomethyDurea, and 0.4-0.5 part by weightof the rubber hydrocarbon of Z-mercapto-4-methyl-thiazole-5- carboxylicacid.

7. The method of vulcanizing natural rubber which comprises heatingnatural rubber, sulfur, 0.1-0.6 part by weight of the rubber hydrocarbonof diphenyl guanidine, and 0.1-0.7 part by weight of the rubberhydrocarbon of Z-mercapto-4-methyl-thiazole-5-carboxylic acid.

8. The method of vulcanizing natural rubber which comprises heatingnatural rubber, sulfur, a vulcanization accelerator mixture of 005-01part by weight of the rub:- ber of tetramethyl thiurarn monosulfide and0.1-0.2 part by weight of the rubber hydrocarbon of1,2-bis(benzothiazolyl mercaptomethyDurea, and 0.4-0.5 part by Weight ofthe rubber hydrocarbon of Z-mercapto-4-n1ethy1-thiazole-5-carboxylicacid.

Jones Feb. 22, 1944 DAmico May 22, 1956

1. THE MOTHOD OF VULCANIZING A SULFUR-VULCANIZABLE RUBBER WHICHCOMPRISES HEATING A SULFUR-VULCANIZABLE HYDROCARBON RUBBER, SULFUR, ANDA MIXTURE OF DIFFERENT ACCELERATORS ONE OF WHICH IS 0.1-4.0 PARTS BYWEIGHT BASED ON THE RUBBER HYDROCARBON OF A COMPOUND SELECTED FROM THEGROUP CONSISTING OF A MERCAPTAN AND ITS SALTS, SAID MERCAPTAN HAVING THESTRUCTURE